1. Field of the Invention
The present invention relates to a humidity sensor and a method of manufacturing the same, and more particularly, to a humidity sensor which has high sensitivity and quick response properties, consumes less power to heat, has fast heating and cooling properties and high durability and is manufactured in an ultra small size, and a method of manufacturing the humidity sensor. The humidity sensor may use various kinds of materials to form a humidity sensitive layer, and may be mass-produced at low production costs in a simple process.
2. Discussion of Related Art
Recently, with growing interest in a ubiquitous environment for obtaining information about things centered around humans, there is an increasing need to develop high-performance sensors capable of obtaining exact and various information in a short period of time.
Among them, a primary core sensor providing ubiquitous service may be a sensor capable of obtaining surrounding information including daily information such as weather conditions. Specifically, daily information such as humidity is the most useful primary information in real life, and a high-performance humidity sensor is needed to realize such ubiquitous service.
This humidity sensor may be used individually or together with sensors obtaining different kinds of information to be applied to logistics service, transport service and a transportation network, thereby improving related services. In addition, it may be applied to production and factory managements and construction of an environmental network in agriculture, fisheries, livestock industries, distribution industries and manufacturing industries, and also applied to home automation, office networks and building control in order to create comfortable housing space or to prevent other harmful industrial environments.
To apply the various services, studies on increasing sensitivity, reducing size and production costs and ensuring reliability for a humidity sensor are under way. In a structural aspect, a humidity sensor has recently evolved into an ultra small-sized micro sensor mainly utilizing microelectromechanical system (MEMS) technology due to the application of semiconductor processing technology from a conventional ceramic sintered or thick film structure.
However, today, the most widely used measurement method for the humidity sensor is a relatively simple method of measuring humidity by measuring electrical characteristics when moisture is adsorbed on or released from a sensitive layer generally formed of an organic material such as a polymer or an inorganic material such as ceramic. Usually, the humidity sensor using such a sensitive layer includes only a humidity sensitive layer and a sensing electrode, and quantifies humidity by detecting a property change of the sensitive layer according to the humidity.
However, the conventional humidity sensor using the sensitive layer has low sensitivity since it uses a planar electrode, and responds with high hysteresis in response to its surroundings. In addition, since the sensitive layer is difficult to reset to an initial state, its response time is several minutes. In the conventional humidity sensor, a large drift in performance is shown due to condensation of moisture on a sensor electrode or a sensitive layer, and durability is decreased due to high stress generated in the sensor structure when humidity is high.
In order to solve these problems, a novel structure has been recently designed, and efforts to facilitate reset of the sensitive layer to the initial state by appropriately heating the sensitive layer using a heater, minimize hysteresis and increase sensitivity are being made. The heater used for the humidity sensor directly affects the sensitivity, hysteresis, reset time, response time, main factors used to measure a performance drift and durability, so that exact and fast temperature control is required for the heater.
Meanwhile, micro humidity sensor devices studied so far have used at least 3 to 10 pattern masks to form functional elements including a thermal insulation structure, and have been manufactured by sequentially stacking the functional elements in a vertical direction on a substrate through a repeated process of thin film deposition, photoresist coating, micro patterning, and thin film etching. Accordingly, when the micro humidity sensor device is manufactured by a semiconductor process, it requires high production costs and a long manufacture period, leading to a decreasing yield. In order to effectively heat a humidity sensitive layer, a micro heater uniformly heating a local corresponding region is suitable, and most of all, this structure should consume low power. In addition, since the humidity sensor should be stably operated for several years, decrease of durability due to thermal stress applied to a sensor structure as the humidity sensor is repeatedly heated and cooled at predetermined temperature by a heater must be prevented.
Various materials for a humidity sensitive layer having an essential role in the humidity sensor have been studied, which include an organic material such as a polymer, a metal oxide, an inorganic material such as ceramic or semiconductor, and a nano material such as nano wires or nano tubes. Generally, a humidity sensitive layer is formed by directly micro-patterning a material for a sensitive layer on a surface of a sensor device using pattern masks, or by micro-aligning a sensor device and then dropping a source material. However, these methods have a problem in developing a unique forming process for a humidity sensitive layer material to be used. Thus, technology capable of easily forming a micro pattern at a specific position on the humidity sensor device is needed to use various materials as the humidity sensitive layer.
A conventional humidity sensor using micro processing technology and problems of the humidity sensor will be described below.
FIG. 1 is a cross-sectional view of a conventional humidity sensor.
The conventional humidity sensor of FIG. 1 is designed to increase the lifespan and reliability of the humidity sensor by reducing a change in characteristic of a sensing electrode according to the time. The conventional humidity sensor has a typical structure of a humidity sensor in which a cavity 102 is formed by anisotropically wet etching a back side of a silicon substrate 100 by its thickness through bulk micromachining, and a heater 110, a sensing electrode and a humidity sensitive layer are formed on a membrane 106 surrounded by insulating layers 104 and 108.
The conventional method using wet etching of silicon to manufacture the humidity sensor is relatively simple. However, it cannot control the depth and shape of the cavity 102, both sides of the substrate must be processed, and production costs are raised as the device's die becomes larger. Moreover, according to this method, a support layer is formed only of the insulating layer 104 in a suspended structure, so that it is vulnerable to repetitive thermal shock and also weak in structure.
FIG. 2 is a cross-sectional view of another conventional humidity sensor.
The conventional humidity sensor of FIG. 2 is designed in a structure capable of minimizing moisture condensation, and thus stably measuring humidity. The humidity sensor uses two substrates, in which a resistance heater 204 is formed on a lower substrate 200 by printing, a humidity sensitive element 206 is formed on an upper substrate 202 and then the upper substrate 202 is mounted on the lower substrate 200. During operation, the humidity sensitive element 206 on the upper substrate is heated to a predetermined temperature of a dew point or higher using the resistance heater 204 on the lower substrate 200 to prevent moisture condensation on the sensor.
Since the heater is formed by special patterning such as printing, and one substrate is mounted on the other substrate, so that they cannot be formed by a standard semiconductor process, the humidity sensor having such a structure is manufactured at high production costs, has low yield and is difficult to form in a small size. Moreover, due to the presence of the resistance heater 204 on a bulk substrate, thermal insulation cannot be sufficiently performed, and thus great amounts of heat are lost and power consumption is increased.
FIG. 3 is a cross-sectional view of still another conventional humidity sensor.
The conventional humidity sensor of FIG. 3 has a structure in which a cavity 310 and a membrane are formed by wet etching a back side of a silicon wafer 308 through bulk micromachining, a stress sensor 306 such as a piezoelectric material is formed thereon, a humidity sensitive layer pattern 304 is separately formed on a base 302 capable of alignment, and these two structures are aligned and assembled on another substrate 300 to allow the humidity sensitive layer pattern 304 to be in contact with the membrane in the cavity 310 formed in the silicon wafer 308. Accordingly, the humidity sensor using mechanical transformation in which a property change of the humidity sensitive layer pattern 304 according to the humidity turns into a stress change of the membrane is realized.
The method of manufacturing a humidity sensor described above cannot solve fundamental problems of humidity sensors, such as hysteresis, because a heater is not included in the humidity sensor formed in a microstructure. In addition, the humidity sensor is manufactured by wet etching, so that the depth and shape of the microstructure cannot be controlled, and the humidity sensor is manufactured by aligning and assembling several structures, so that the production costs are increased.
Meanwhile, most known methods of manufacturing a micro humidity sensor generally use a silicon substrate having very high thermal conductivity. Accordingly, in order to reduce thermal loss, an etched pit or groove is formed in a sensor structure by bulk micromachining to form a suspended structure separated from a substrate, and a heater, an insulting layer and a humidity sensitive layer are sequentially formed on this suspended structure, thereby somewhat reducing heat loss. However, since this process must be performed on both sides of a substrate, it requires high production costs, and since this is a wet etching process using crystal orientation of the substrate itself, the depth and shape of a thermal insulation structure cannot be freely controlled. Moreover, this method has a limitation in manufacturing a small-sized sensor device and has difficulty in compatibility with a standard CMOS semiconductor process, due to the usage of a special etchant such as potassium hydroxide (KOH).
Therefore, there is a need for a humidity sensor having improved characteristics and manufactured by a simple and economical process, and a method of manufacturing the same.